Approximately one out of every ten females born today in the U.S. will develop breast cancer. This high incidence has prompted the need to understand the biochemical basis for susceptibility to this disease. Ovarian steroids are known to have an essential role in this disease but it is not understood how these compounds function in stimulating growth of either the normal gland or breast carcinoma. In one study just recently completed, we investigated estrogen-dependent mammary gland development (ductal morphogenesis) for local expression of members of the epidermal growth factor (EGF) family. Transcripts of EGF and TGF-alpha (transforming growth factor-alpha) were identified and immunolocalization studies revealed that these factors are made in separate cell populations of the gland. Both polypeptides were able to stimulate growth of the "regressed" gland of ovariectomized mice in vivo. These findings suggest that estrogen-stimulated ductal growth is mediated by a local EGF-like polypeptide and that synthesis of the EGF-receptor is not apparently dependent directly on ovarian steroids. Inhibitors of the EGF-receptor pathway are currently being evaluated for inhibition of ductal growth in vivo. We are also investigating the functional relationships between ductal epithelial cells in adult animals that contain EGF and similar ductal cells that contain estrogen and progesterone receptors. Comparisons will be made between the rodent gland and human breast tissue obtained by reduction mammoplasty. A class of proteases (kallikreins) that might be important in the processing of the EGF precursor (about 140 kD) has been detected in the lactating mammary gland which synthesizes the EGF precursor. These enzymes are currently being evaluated for their capacity to be hormonally regulated and also for their role in converting the membrane-bound precursor to the active EGF peptide. In another study, we are investigating murine mammary preneoplastic lesions for ectopic expression of prolactin and related proteins that might account for the immortalization and phenotypic properties of these neoplasms.